The obligate intracellular pathogen Chlamydia trachomatis is the most common cause of sexually transmitted disease in the developed world, causing both overt disease and infertility, and is also the leading cause of preventable blindness worldwide. Infection with C. trachomatis results in a specific immune response against the organism, and CD8+ cytotoxic T-lymphocytes (CTL) specific for C. trachomatis can be cultured from infected mice. Adoptive transfer of cultured CD8+ T-cells into infected mice reduces the number of organisms found in the spleens of these animals. The experiments in this proposal are designed to determine which Chlamydia gene products are recognized by these Chlamydia specific murine CD8+ T-cells. Three independent approaches will be used to identify these antigens. Once the C. trachomatis gene products responsible for priming the CD8+ T-cell response are identified, they will be introduced into vaccinia virus and tested for their ability to stimulate protective T-cells in a vaccine strategy. Because these antigens stimulate CD8+ T-cells, it suggests that they have access to the cytosol of host cells during the C. trachomatis developmental cycle. Experiments to characterize the developmental regulation and subcellular localization of these proteins will be carried out as a first step in understanding the role these proteins may play in C. trachomatis pathogenesis. Additional experiments will focus on the mechanism by which CD8+ T-cells protect against Chlamydia infection. Interferon-g (IFN-g) appears to be the primary effector mechanism used by these T-cells to protect against C. trachomatis infection. IFN-g release by CD8+ T-cells could protect mice against C. trachomatis infection by stimulating the antimicrobial activity of macrophages or by directly inhibiting the replication of C. trachomatis. Experiments to clarify the mechanism by which IFN-g produced by T-cells mediates protection will use radiation bone marrow chimeras in which either the hematopoietic cells (including macrophages) or the resident cells are unable to respond to IFN-g as a result of a disruption in the IFN-g receptor gene. A better understanding of the immune response to C. trachomatis and the development of a vaccine would have a pronounced effect on worldwide morbidity resulting from theseinfections.